Signaling system



Filed Sept. 29, 1934 4 Sheets-Sheet lllw ms: u

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SIGNALING SYSTEM SIGNAL AMPLIFIER RECEIVER INVENTO"? Ra Dlfell S'IGNHLGENERATOR Nov. 24, 1936. v R L 2,061,734 SIGNALING SYSTEM Filed Sept.29, 1954 4 Sheets-Sheet 2 V'R/INSMITTEW ATTORNEY ,u INVENTOR Nov. 24,1936.

R. D. KELL 2,061,734

S IGNALING SYSTEM Filed Sept. 29, 1934 4 Sheets-Sheet 3 z- AvmvA To GRIDor 78 runs 34 (FIG. 2.)

I/VVENTOR DERIVATIVE D. N IMPULSE GENERATOR HND case-ran 7; BY W YATTORNEY 4 Sheets-Sheet 4 R. D. KELL SIGNALING SYSTEM Filed Sept. 29,1954 Nov. 24, 1936.

Patent d Nov. 24, 193.6

NITED STATES SIGNAILHNG SXSTEM Delaware Application September 29, 1934,SerialNo. 746,086

13 Claims. i. 250-6) 8, 1932, to Finch, disclose systemsof that sort.

My invention, however, is directed more particularly to apparatus forimproving the ratio between signal strength and the amplitude ofinterfering waves or sources of noise effects.

Accordingly, it is among the objects of my invention to provide asignaling system in which a carrier may be so modulated for thetransmission of intelligence that the signals may be received andtranslated with a high degree of fidelity and the minimum of noiseaccompaniment.

Further objects of my invention include the provision of suitableapparatus for transmission and reception of intelligence in whichnonlinearities of the amplifiers may be neglected without causingdistortion.

At the present time it is the general practice to amplitude modulate thecarrier wave. The method of modulating a carrier of constant amplitudeas proposed by Heising and by Finch, above cited, and possibly byothers, has not been generally adopted and practiced, perhaps owing tocertain operational difliculties that were encountered. it is,therefore, a further object of my invention to overcome suchdifficulties by the use of improved structural elements in thesystem.

The foregoing and other objects and advantages of my invention will bemore clearly understood upon reference to the following detaileddescription when read in connection with the accompanying drawings, inwhich Figure 1 shows a circuit diagram of one embodiment of a radiotransmitter suitable for voice modulation of a carrier wave of constantamplitude,

Fig. 2 shows a circuit diagram of a radio receiver suitable for thetranslation into intelligence of a constant amplitude modulated car- Iprefer to employ in transmitting voice modulated signals on a constantamplitude carrier,

Fig. 4 shows a circuit detail useful for translating signals which arereceived according to a modification of my invention,

Figs. 5 and 6 illustrate diagrammatically the forms of certain waveswhich are utilized in the carrying out of my invention} Fig. '7 shows acircuit detail of a receiver when used.- according to anothermodification of my invention and,

Fig. 8 shows a schematic illustration of another modiflcation of myinvention.

It may be stated at the outset that in the description to follow, myinvention will be illustrated as having particular applicability to thevoice-modulation of a carrier wave and the demodulation thereof. It willbe apparent, however, that the apparatus is not limited to this use,because the same principles apply to the modulation of a carrier eitherfor the transmission and reception of signals of any type, Whetherrepresenting sound, pictures, telegraph code, facsimiles or otherintelligence.

For faithful reproduction at the receiver of the modulated signals atthe transmitter, it has heretofore been considered essential that allthe circuit elements should be linear; that is, that they should respondin proportion to the intensity of the applied signal. Under thislimitation it has been known for a number of years that anydiscrimination between desired signal and noise entering the signalingsystem must be the result of selectivity of the circuits involved. Ifthesignal occupies the minimum frequency range necessary fortransmission of the desired intelligence and the selectivity of thecircuit used is such that it passes this signal frequency rangeireelybut suppresses disturbances of all other frequencies, no furtherimprovement in the ratio of signal to random noise can be hoped for,except of course,

by increasing the intensity of the transmitted signal.

The possibilities of improvement of the ratio of signal intensity tonoise intensity I have found to lie in a departure from ordinary methodsof amplitude modulation. I have found also that these possibilities maybe extended by the use of non-linear circuits having an output by nomeans proportional to the input.

Briefly, my system contemplates the transmission of certain timingmarks. The transmission of intelligences may be had without in the leastdepending upon amplitude modulation of the carrier. The improvement inthe ratio of signal intensity to noise intensity, therefore, is obtainedlargelyby virtue of the fact that these timing marks can be shifted overa wide range by the desired signal, but only over a definite narrowrange by any noise of less amplitude than the signal. It is admitted,however, that any interference causing noise of greater amplitude thanthe signal is not suppressed in the operation of my system, but, becauseI am able to greatly intensify the amplitude of the transmitted signalswithout increasing the amount'of power radiated. it will be apparentthat the signal-to-noise ratio is in large part improved over what ispossible when using the generally accepted present-day methods.

Referring to Fig. 1, I show a transmitter network including electricdischarge tubes i and 2 which are employed as a multi-vibrator for thepurpose of interrupting the carrier frequency at a constant superaudiblerate; for example, 30,000 interruptions per second. These interruptionswill hereinafter be referred to as a dot-frequency.

The network for causing the electric discharge tubes I and 2 to operateas a multi-vibrator is conventional. It comprises an input circuit fortube I having a grid leak I connected with the grid 8, and an inputcircuit for tube 2 having a grid leak 9 connected with the grid Ill. Theanode circuits for these tubes are supplied with direct-current from anysuitable source such as I I which may be fed through the resistors I2-I2to the anodes I3 and I4, respectively. A capacitor I5 is interposedbetween the anode circuit of tube I and the grid circuit of tube 2.Another capacitor I6 is likewise interposed between the anode circuit'oftube 2 and the grid circuit of tube I. The interruption frequencyproduced by this multi-vibrator may be utilized in an output circuit,which is conveyed through the resistor I1 and the capacitor I8 to thegrid I9 of the electric discharge tube 3. In order to maintain a properbalance between the two tubes I and 2, in view of the unsymmetricaldrain of energy into the utilization circuit, a variable capacitor 20may be inserted between the grid I0 and ground.

The output from the multl-vibrator is in the form of a rectangular wavethe pattern of which is represented at a in Fig. 6. The repetitionfrequency of the impulses in this wave is represented as fa, the timevalue of which is preferably made up of two equal elements, one being apositive impulse and the other a negative impulse. It is the function ofthe resistor II and capacitor 20a to convert these impulses intoimpulses of triangular or saw-tooth pattern, as shown at b in' Fig. 6.When, in addition to the impulses id, there is superimposed a voicefrequency fv, the pattern will become as shown at d in Fig. 6. Asignaling wave may be impressed directly upon the dot-frequency fa fromany desired source, such as the signal generator 2 I. This may be amicrophone or a modulator of any desired type whether used for voice orpicture transmission, or for the transmission of any other intelligence.The voice frequency fl) is amplified by the amplifier 22 and impressedthrough the transformer 23 between the cathode 24 of tube 3 and ground.The secondary 25 of the transformer 23 offers a high impedance toalternating currents of the dotfrequency, which are therefore shunted toground through the low impedance resistor 30. The impress of a voicefrequency upon the secondary 25, in circuit between the cathode 24 andground is such as to swing the bias of the grid I9 to a considerabledegree. The grid biasing circuit may be traced through a resistor 28 toa suitable point on the potentiometer 21, where a negative potential issupplied by the battery 28 with respect to ground. A capacitor 29 isalso provided for filtering purposes, as is usual.

The fixed bias of the tube 3 is adjusted to a certain value which, inthe absence of output from the multi-vibrator, would suppress thetransmission of negative waves from the modulator 2I. Referring to thecurve at in Fig. 6, it will be seen that only that portioncf thecomposite wave 3| which lies above the base line will be transmittedthrough the tube 3. The cut-off point of the tube characteristicsuppresses the under portions of this zig-zag curve.

The output of tube 3 may be impressed across the capacitor 32 and upon atube 4, the bias of which is so fixed as to limit the plate current forproducing square-topped waves. These waves may then be successivelyimpressed upon the tubes 5 and B for further leveling off their peaksand for amplification. Thence they are impressed upon a radio frequencyoscillator and amplifier 33 for transmission in the usual manner.

In the absence of input potentials to the system from the signalgenerator 2 I, the carrier is interrupted at a constant superaudiblefrequency, fa, determined by the adjustment of the multi-vibrator. Whenthis dot-frequency is unmodulated the carrier assumes a pattern theenvelope of which may be represented as shown at c in Fig. 6.

The time intervals when the carrier is "on are equal to the timeintervals when the carrier is off". The action of the tubes 3 to 6inclusive is such, however, as to vary the on and off" intervals inrelation to one another according to the bases of the triangles of thecurve III. The voice modulated dot-frequencies thus produce a carrierwave envelope such as shown at e in Fig. 6. During a single cycle ofvoice frequency ,fo the on time of the carrier will vary from a maximumto a minimum. The degree of variation represents the percentage ofmodulation. In Fig. 6 the effects of a high percentage and a lowpercentage of modulation are shown by the differences between theheights of the curves at and g and the corresponding breadths of thecarrier envelopes e and h. The patterns (1 and e are intended torepresent a modulation, whereas the patterns g and 11 representsubstantially a 45% modulation. When the voice frequency ishigher-pitched than as shown in the examples at d and g, a wave offrequency jw may be produced as shown by the pattern 1' in Fig. 6. Thecarrier envelope produced by this wave is shown at in Fig. 6.

From the foregoing it will be seen that the power radiated is a functionof the modulation just as in normal amplitude modulation. In thereceiver the carrier is detected either directly or heterodyned andamplified before detection, as is well known in the art. The output ofthe detector contains the impulses of current together with interferingimpulses which would produce undesirable noise and hiss. The transmittedcarrier plus the interfering waves may be represented as shown at k inFig. 6. The signal at the output of the detector still has theinterference superimposed upon the impulses, for which reason it isdesirable to employ a limiting tube such that the center of the impulseswill not be passed. Such a tube is'shown at 34 in Fig. 2, and it will beunderstood that it is so biased as to remove the undesired interferingwaves at 76 pulses.

least in part and thereby to produce a wave of the form 1 in Fig. 6. Afurther limiting tube 85 may then be employed for removing the remaininginterference constituting the ragged wave crests, so as to produce awave of the form t in Fig. 6. When the plate current of tube assumesthis pattern it is enabled to control any signal responsive device ofordinary type, such as that indicated at 36 in Fig. 2, the energy beingtransmitted thereto, if desired, through a capacitor 37 and audiofrequency transformer 38.

Still further advantages of vmy invention by way of suppressing theeffects of interference may be-obtained upon employing a modificationthe embodiment of which will now be described. In this embodiment theintelligence is transmitted by marking only the edges of the originalim- The network shown in Fig. 3 may be used for this purpose. In placeof the tube 6 of Fig. 1 this network is interposed and comprises tubes,40, M, 42 and 43. The output from tube 5 may be impressed across thecapacitor 44 upon the grid of the tube 40, thereby producing variationsin plate current which develop a voltage drop across the inductance 45.This voltage is the derivative of the impulses. When the direct currentflowing through the resistor M is steady, no eifefct is produced uponthe grids of tubes MI and M. The signalling impulses, however, produce avoltage wave such as shown at n in Fig. 5. Here it will be seen that atthe commencement of I each dot, as modulated in the manner hereinbeforedescribed, there will be a sharp peak of positive value, and at thetermination of each dot there will be a similar sharp peak of negativevalue. The time elapsing between positive and negative peaks isrepresented as the equivalent of a marking impulse m. The time intervalbetween negative and positive impulses is likewise represented as aspacing interval 8. These positive impulses may be transmitted throughthe tube 4i directly to the transmitter it. The negative impulses areinverted through the cooperation of tubes M and 43 and likewiseimpressed upon the transmitter M. The modulation therefore assumes apattern such as shown at p in Fig. 5. These modulations result incontrolling the on" and off time of the radio frequency oscillator andamplifier output such that the carrier is on" only during the very shortintervals of the sharp peaked impulses of the pattern 1). The envelopeof the carrier will be represented as shown at q in Fig. 5, wherealternate intervals m betweenthe impulses correspond with markingintervals cf the original signal, while intervening intervals representthe spaces s of the original signal.

- The operation of tubes ii, tt'and it is as follows: Both tubes M andit receive the same signals. The cut-ofi point of tube M is so adjustedthat it will transmit the positive impulses. Tube 42 will transmit boththe positive and negative impulses, and tube M will transmit onlypositive impulses. Tube 42 reverses the phase of the signals, impressingthe originally negative impulses positively upon the grid of tube M,which then operates alternately with respect to tube M. Tube- H3 is sonegatively biased that it derives no effect from tube-42 when thepositive impulses are being transmitted by tube M. The combined actionof tubes 4 I, 2 and 43 is to transmit impulses at the commencement andat the termination of each marking intervalm. Hence, the carrier wavemay be controlled by uniform impulses such as those shown in the diagramp and the carrier envelope is constructed as at q.

If the output from the transmitter is controlled as described in theforegoing paragraph, the "on" time of the impulses may be as little as10% and the "off" time as much as 90%. A transmitter having a givenrated continuous power output will, therefore, be enabled to transmitthese discrete impulses at substantially ten times the amplitude ofcorresponding, continuous impulses. I have shown how, at the receiver,interference of less amplitude than that of the carrier may besuppressed. Assuming that the amplitude of this interference is notincreased, it will be seen that the greatly increased amplitude of thecarrier impulses provides an improved ratio thereof to the noise levelrepresented by a factor of approximately 10.

The apparatus and the method employed at the receiver make possible astill further gain in the ratio of signal response to noise response.This will be better understood upon observing that the raggedness of thewave crests due to interference may be removed by the operation of thelimiting tube 35; and interference received when the carrier is keyedoff may be removed by the operation of the limiting tube M.

Transmission of sound by the means described in the foregoing paragraphsis, of course, unintelligible to the conventional radio receiver, sincethe power radiated is a constant. It is necessary, therefore, totranslate the signals'by some such means as shown in Fig. 4. The networktherein represented comprises tubes ht, 5i and b2. These tubes may beplaced between the receiver it and the network of tubes it and lit,shown in 3 Fig. 2. The input energy of the signals as derived from thedetector of the radio receiver Mi may be impressed across the capacitorit to an intermediate tap M between resistors 5t and ti.

Tubes iii and M are interconnected so as to form substantially amulti-vibrator operating at a frequency below that at which it may bedriven when signals are impressed thereon. The bias on tube till is suchthat it is inoperative at the start of a cycle. The first impulse causesthe grid of tube hi to go positive, which in turn causes the plate oftube 52 also to go positive. The two tubes ti and ti? therefore go intothe limiting condition. The decrease in plate current in tube 52 causesthe voltage on the grid of tube fit to become less negative. This resultis attributed to the rise of potential at the point hit on the resistortiin the anode circuit of tube 52. Tap it connects with a point W on thegrid leak circuit for tube 5b. This point it is intermediate theresistors W and the grid biasing battery M where the cathode-to-gridcircuit includes resistor 5%, battery M and a further resistor ti. Sincetube b? is now blocked and the potential at the point 58 has been raisedto bring the bias of the tube til almost but not quite to the cut-offpoint, a succeeding impulse impressed on the input circuit through theresistor hill and the capacitor M will control the tube bliso as to passplate current through the output circuit resistor b t. This conditionimpresses a negative impulse upon the grid of tube M, which blocks thistube and causes the grid oftube 52 to go positive. The network of themulti-vibrator is thus tripped to the other limiting condition. Thisaction may, therefore, be repeated to reconstruct the original impulses.then be seen to constitute signals of varying length, as shown by themarking-intervals m The energy so derived may and spacing intervals 3 inthe pattern 1' of Fig. 5. Such a wave may be impressed upon a signalresponsive device of any suitable type, such as aloud speaker oi" aradio receiver.

A further modification of my invention 'may be made by employingsuitable means to produce impulses at the transmitter which are ofunsymmetrical saw-tooth pattern. These may easily be constructed in anywell known manner. When modulated they will take the form of pattern aas shown in Fig. 5. The "on" and "of!" intervals oi the carrier wavewill be controlled in accordance with the bases of the triangles of theu-pattem. Here the modulated impulses come on" at variable times butterminate equidistantly in point 01' time. The results produced at thereceiver, however, may be utilized in the same manner as hereinbefore..described with respect to the first modification.

They may also be used in accordance with the second modification it themeans shown in Figs. 3 and 4 are brought into play. In this case,however, it is not necessary to transmit the stationary edge of eachimpulse. They may be removed by omitting tubes 42 and 43. Thesestationary edges may be supplied at the receiver by an oscillator H (seeFig. '7) running at approxi-' mately the same frequency as the originalimpulses. These two sets of impulses may then be used as hereinbeforedescribed to reconstruct the original impulses.

Tl'reoperation of the network shown in Fig. 7 is as follows: Impulsesfrom the detector are impressed upon the grid of tube 12 causing it togo positive. Anode current will fiow in the resistor 13, dropping thepotential at the point 14 and impressing a negative surge across thecapacitor 16 and -upon the grid of tube I8, causing this tube to beblocked. The anode potential across resistor ll then rises and transmitsan impulse across the capacitor 18 to the grid of tube 34. (Fig. 2)which operates as hereinbefore described. A positive impulse is alsoimpressed across the capacitor 19 causing the tube 12 to go to thelimiting condition. Next, a positive impulse produced and selected bythe device under control of the local oscillator H is impressed upon thegrid of tube 16, causing it to go positive and tripping the operation oftubes 16 and 12 as is well understood in respect to multivibrators.These tubes will, therefore, trip one way at variable times in responseto the received signals and the other way at regular times in responseto the device 86. thus rebuilding a wave of the pattern t (Fig. 6). r

Due to the fact that the signaling intelligence is represented only bythe time intervals intervening between impulses, suitable amplifiers maybe constructed in which non-linearities are neglected. Thus distortiondue to curvature of vacuum tube characteristics becomes non-existent,thus increasing the possibility of using gas-filled tubes of any wellknown type so as to produce high audio power, if desired.

In Fig. 8, a modification of my invention has been illustrated inschematic form. The asymmetric saw tooth wave generator iili may be ofthe type illustrated in U. S. Patent 1,887,237

which issued on November 8, 1932 to J. L. Finch. The modulator I03 hasbeen previously described in connection with thermionic tube 3 ofFig. 1. The wave shaper I 05 has been illustrated by this figure, tube40 and its circuits is an impulse generator. The inverter I" has beendescribed in connection with tuba l2, 4! oi. Fig. 3. The carrier wavegenerator ill may be any type of continuous wave generator; such as, athermionic tube oscillator.

The receiver H3 may be a superheterodyne, or a tuned radio frequencyamplifier. Such amplifier is disclosed in U. 8. Patent 1,173,079 toErnst Alexanderson. A wave shaper network M6 for the receiver has beendescribed in detail in connection with Fig. 4. The current limiter Ii'imay be of the type shown in U. 8. Patent 1,468,116 which issued onSeptember 18, 1923 to Irving Langmuir. The signal indicator Mil may be aloudspeaker, cathode ray tube, facsimile reproducer or the like.

Instead of inverting alternate ones of said impulses, the uniformlytimed or uniform interval impulses may be suppressed by omitting tubesat and 43 of Fig. 3. In this case, the irregularly spaced impulses IIIare used to key the transmitter. The regularly spaced impulses 123 aresupplied by an impulse generator I25, located at the point of reception,which is operated at the same frequency as the original impulses. Thisimpulse generator may correspond to tube st of Fig. 3.

Although I have shown several specific embodiments of my invention, itis to be understood that these are merely illustrative and that othermodifications may be made without departing from the spirit and scope ofthe invention as defined in the appended claims.

I claim as my invention:

1. A signaling system comprising a carrier wave source, a source ofsignals, a source of dotimpulses of supersonic frequency, means forkeying the carrier wave on and off at an average frequency equal to thatof said supersonic impulses and means under control of said source 01signals for varying the time intervals between successive moments ofkeying the carrier wave on while maintaining constant the duration ofeach on" impulse.

2. A signaling system in accordance with claim 1 and comprisingreceptive means including a network for translating signals representedby impulses of uniform amplitude and uniform duration into signals ofuniform amplitude and variable duration.

3. A signaling system comprising a carrier frequency generator, meansfor producing a succession of discrete impulses for modulating theoutput of said generator, said impulses being of substantially uniformamplitude and duration, 9. source of signals 01' lower frequency thanthe recurrence period of said impulses, and means for causing saidsignals to vary the time intervals elapsing between successive ones ofsaid discrete impulses.

4. A system in accordance with claim 3 comprising additional apparatusfor reception of the carrier wave, said apparatus having means forconstructing a wave of substantially rectangular pattern having fiattops the time components of which correspond substantially to alternatetime intervals elapsing between successive modulations of the carrierwave, and said pattern having fiat bottoms the time components of whichaccuse ,correspond substantially to intervening time intervals, wherebysaid signals are reconstructed.

5. In a system for the transmission and reception of signal-modulatedcarrier-wave impulses of constant frequency, constant amplitude anduniform duration, a multi-vibrator for producing a rectangular wave,means for translating said rectangular wave into a wave of triangularpattern, modulating means for varying the amplitude of said triangularwave, means for truncating the peaks and valleys of said triangularwave, means for deriving impulses of constant amplitude, uniformduration and spaced apart in accordance with the moments of voltageshift of the wave originally triangular but now truncated in pattern,means for modulating said carrier wave under control of the last saidimpulses, means for receiving and detecting said impulse-modulatedcarrier wave, and means including a multi-vibrator adapted to be drivenby the detected impulses for reconstructing a wave the pattern of whichcorresponds substantially to the wave of truncated triangular pattern.

6. A system embodying the features recited in claim and characterized inthat the means for modulating said carrier wave has in combinationtherewith a device'for accepting alternate impulses and rejectingintervening impulses corresponding to the moments of voltage shift ofthe truncated triangular wave, and the means for receiving and detectingsaid impulse-modulated carrier wave has in combination therewith adevice for supplying substitute impulses in place of the rejectedintervening impulses.

7. A system embodying the features recited in claim 5 and characterizedin that the means for translating the rectangular wave into a wave oftriangular pattern is adapted to produce an asymmetrically triangularwave and the means for modulating the carrier wave is adapted to vacceptonly the impulses derived from voltage rier wave with said discreteimpulses now all of one polarity, transmitting and receiving saidmodulated carrier wave, and rebuilding at the point of reception a wavecorrespondingto the flat-topped and flat-bottomed wave aforementioned.

9. The method as set forth in claim 8 including the further steps ofleveling off the peaks and valleys of the wave rebuilt at the pointofreception so as to derive a wave substantially free from undesirablenoise-producing components.

10. The method of signaling which comprises producing an asymmetricsaw-tooth wave, modulating said saw-tooth wave, deriving therefrom afiat-topped and flat-bottomed wave, deriving, further, a series ofalternately positive and negative discrete impulses separated by timeintervals substantially proportional to the flat tops 'and flat bottoms,respectively, ofthe wave so formed,

inverting alternate ones of said discrete impulses. generating a carrierwave, keying said .carried wave on for predetermined moments and "off"for irregular time intervals elapsing between the discrete impulses,transmitting and receiving said carrier wave, and rebuilding at thepoint of reception a wave corresponding to the flat-topped andflat-bottomed wave aforementioned.

11. The method as set forth inclaim 10 including the further steps ofleveling off the peaks and valleys of the wave rebuilt at the point ofreception so as to derive a wave substantially free from undesirablenoise producing components.

12. The method of signaling which comprises producing an asymmetricsaw-tooth wave, modulating said saw-tooth wave, deriving therefrom aflat-topped and fiat-bottomed wave, deriving, further, a series ofalternately positive and negative discrete impulses separated by timeintervals substantially proportional to the flat tops and flat bottoms,respectively, of the wave so formed, suppressing those of said discreteimpulses which occur at uniform intervals. generating a carrier wave.keying said carrier wave by the irregularly timed of said discreteimpulses, transmitting and receiving said carrier wave, supplying at thepoint of reception discrete impulses at uniform intervals substantiallyequal to the intervals of said suppressed impulses. and rebuilding atthe point of reception a wave corresponding to the flattopped andfiat-bottomed wave aforementioned. .13. The method as set forth in claim12 including the further steps '"bf leveling off the peaks and valleysof the wave rebuilt at the point of reception so as to derive a wavesubstantially free from undesirable noise producing components. RAY- D.vKELL.

